1. Field of the Invention
The present invention relates in general to deflection units for in-line type cathode ray tubes and refers, in particular, to those deflection units that comprise a coil body including substantially straight grooves for receiving wound coil wires.
2. Description of the Related Art
In color picture tubes of the in-line type, an electron beam generating system is designed for generating three coplanar electron beams that converge on the screen. The deflection unit which is arranged around the neck portion of the picture tube is used for deflecting the electron beams from their normally straight path into the one or other direction so that the beams impinge upon selected points of the screen to produce a visual signal. With a suitable time variation of the magnetic deflection fields the electron beams can be deflected upwards or downwards and to the right or left side across the screen.
In in-line type color picture tubes three electron guns are positioned side by side. To deflect the generated electron beams into the X- and Y-direction, the funnel-shaped deflection unit mounted on the color picture tube produces deflection fields so that a self-converging picture without north-south raster distortion is obtained on the screen. This funnel-shaped deflection unit consists essentially of a pair of horizontal coils that produce a magnetic field which deflects the beams into the X-direction, a pair of vertical coils for deflection into the Y-direction, a ferrite core which encloses the coils, and correction magnets and possibly soft-magnetic field shapers.
The coil bodies for the pairs of horizontal and vertical coils are normally built up by way of groove winding techniques. A section through such a coil body is shown in FIG. 1a. As becomes apparent from the figure, the coil body comprises a plurality of straight grooves 10, 20, 30 into which a winding wire 40 is inserted during winding of coil. Since the groove extends in straight fashion, the winding wire is also wound along a straight line. The field distribution can be varied by changing the distance or an optionally existing angle between neighboring grooves. FIG. 1b shows one of the used grooves in cross section.
FIGS. 2a and 2b show the field distributions of the horizontal and vertical deflection field along the Z-direction, i.e., in the direction towards the screen. As can be gathered from the figures, the field distribution within the deflection unit is variable. In particular, the field distributions in the front and rear portion of the deflection unit differ from one another. The term xe2x80x9cfront portionxe2x80x9d means the portion of the deflection unit that is facing the screen.
In in-line type cathode ray tubes the two following types of errors can among others be observed: convergence errors and geometry errors.
Convergence errors arise when the different primary color images have no congruent rasters. A special type of convergence errors arises in in-line type cathode ray tubes as coma errors because the raster dimensions of the three electron beams on the screen are different. This is a result of the eccentric position of the outer electron beams relative to the horizontal and vertical deflection fields.
Geometry errors cause a deviation of the raster geometry on the screen from that of an ideal raster independently of possible convergence errors. The cause for geometry errors are the different path lengths of the electrons relative to the individual pixels on the screen.
Therefore, correction mechanisms are known in the prior art that aim at a correction of convergence and geometry errors. This is in particular achieved in that inhomogeneous deflection fields as are illustrated in FIGS. 3a to 3d are produced by complicated winding techniques at specific Z-positions of the deflection unit. These figures show a horizontal barrel field in FIG. 3a, a horizontal pincushion field in FIG. 3b, a vertical barrel field in FIG. 3c and a vertical pincushion field in FIG. 3d. 
However, the use of such pincushion-shaped or barrel-shaped magnetic deflection fields only permits a convergence error or geometry error correction up to a certain degree of accuracy.
It is therefore the object of the present invention to provide a possibility which allows the production of deflection units for mounting on an in-line type cathode ray tube with less convergence and geometry errors.
According to the invention this object is achieved by a deflection unit for mounting on an in-line type cathode ray tube which comprises a coil body with grooves for receiving wound coil wires, the grooves extending in a substantially straight line. The groove wall of at least one groove at at least one location is changed such that the coil wires received in the groove become curved in the area of the changed groove wall.
This selective change in the groove wall permits a finely adjustable and selective error correction which also permits field distributions differing from the field distributions shown in FIGS. 3a to 3d. Moreover, changes in the groove wall thickness can be made at any desired locations of the coil body so that many types of errors, including convergence errors, coma errors and geometry errors, can also be dealt with in a selective manner.
In a preferred embodiment the grooves are separated from one another by groove walls which in the area of the changed groove wall comprise a thickening and possibly a bulge on the opposite groove wall. The winding wires are thereby forced into their defined changed position.
Preferred embodiments of the invention are indicated in the subclaims.